Solar cells are components that convert light into electric energy. Normally, they are made of semiconductor materials that contain areas or layers of different conductivity for positive and negative charge carriers, namely, conductive areas of the n-type or p-type. These areas are referred to as emitters (emitter layer) and absorbers (absorber layer, absorber wafer). Positive and negative excess charge carriers generated in the absorber by incident light are separated at the pn-junction between the emitter and the absorber and can then be collected and discharged by the contact systems that are electrically conductively connected to the appertaining emitter and absorber areas. The excess majority charge carriers from the absorber are collected and discharged by an absorber contact system (majority charge carrier contact system) and the excess minority charge carriers from the absorber are collected and discharged by the emitter and by an emitter contact system (minority charge carrier contact system) that contacts said emitter. Consequently, only those excess charge carriers that reach the contact systems and that do not recombine before that with a charge carrier of the opposite charge contribute to the useful electric performance of solar cells.
Back-contacted solar cells have both contact systems for separately collecting the excess majority and minority charge carriers from the absorber wafer on the side of the solar cell facing away from the light. For one thing, this has the fundamental advantage that only this side of the absorber wafer needs to processed for contacting purposes, while the other side remains unprocessed when it comes to the contacting. If the absorber wafer is of a sufficiently high electronic quality, in other words, if the effective diffusion length of the minority charge carriers is greater than the thickness of the wafer, then the current-dissipating contact systems can be on the back of the solar cell facing away from the light. This especially entails the advantages that, first of all, no shading losses occur due to a contact system that is on the front, thus improving the efficiency of the solar cell, and secondly, the front of the solar cell facing the light can be passivated easily and properly over the entire surface so as to effectively and easily prevent a recombination of the excess charges. Moreover, back-contacted solar cells can be more easily interconnected to form modules and they are esthetically very attractive.
However, conventional back-contacted solar cells entail several drawbacks. Their production processes are usually complicated. Some processes call for several masking steps, several etching steps and/or several vapor-deposition steps in order to create the absorber contact system so that it is electrically separated from the emitter contact system on the back of the wafer. Moreover, conventional back-contacted solar cells are often prone to local short-circuits caused, for instance, by inversion layers between the absorber and the emitter or by inadequate insulation between the contacts, which translates into a reduced efficiency of the solar cell.
One concept of one-sided back contacting puts forward the use of surface elevations as described, for example, in German patent application DE 41 43 083 A1. Here, the first and second contact systems are arranged directly, or else on an insulation layer, on a semiconducting substrate surface having elevations (for instance, in the form of pyramids, cones or cylinders), whereby the elevations have previously been covered, at least in certain areas, with passivation material, and these elevations are subsequently exposed in certain areas so that the contact systems can be applied. German patent application DE 41 43 084 A1 describes first passivating the entire structured substrate surface and subsequently removing the passivation layer in the area of the elevations. German patent application DE 101 42 481 A1 describes arranging these elevations in the form of ribs on the bottom of the active semiconductor substrate and providing each rib flank with a contact system by means of targeted vapor-deposition. Part of this concept is thus to always create elevations on the bottom of the substrate and to then process them in different ways. German patent application DE 10 2005 040 871 A1 describes a back-contacted solar cell in which the emitter contact as well as the absorber contact are electrically insulated from each other by flanks on which a metallization layer that has been previously applied to their entire surface is removed by etching. The n-doped and p-doped areas that are to be contacted are interdigitated either only on the back of the wafer, or else on the front and back of the wafer, whereby the doped area on the front of the wafer extends along wafer channels to the back of the wafer.
Another concept of back contacting for a wafer-based system is point contacting (PC). Here, both contact systems are kept very small in the form of points on the back of the solar cell so as to lower the saturation reverse current and thus to increase the open circuit voltage of the solar cell. U.S. Pat. No. 5,468,652 describes, for instance, point contacting in which the emitter layer arranged on the top of the absorber wafer facing the light is contacted with a contact system on the back of the wafer by point contacts through the absorber wafer. Here, this contact system is arranged so as to be nested side by side with the contact system for dissipating the majority charge carriers of the absorber layer.
U.S. Pat. Appl. 2006/0130891 A1 describes a back-contacted heterojunction solar cell having an absorber wafer and a back emitter layer configured as a surface with punctiform or strip-shaped vias. Additional functional layers can also be provided. The absorber wafer and the emitter layer consist of oppositely doped semiconductor materials and they span a pn-junction.
The emitter layer is contacted by a back emitter contact system that is configured as a surface with punctiform or strip-shaped vias. The absorber wafer is contacted by a back absorber contact system that is configured as an absorber contact layer or as an absorber contact grid with point contacts or strip contacts that extend through the punctiform or strip-shaped vias of the emitter contact layer. The two contact systems lie one above the other on the back of the absorber wafer, which is not intended for exposure to incident light, and they are electrically insulated with respect to each other by an insulation layer. During the production of such a solar cell, first of all, the layers are built up, all the way to and including the insulation layer, which is then appropriately structured in order to create the point contacts or strip contacts of the absorber contact system, for instance, by means of laser structuring. Subsequently, a metal layer is applied so as to fill the previously created structures. A special difficulty encountered with such back-contacted solar cells is the laborious production of the back contacts during which it is absolutely necessary to prevent electric short-circuits.
German patent application DE 2004 046 554 A1 describes a punctiform back contacting of the absorber wafer for a solar cell that is contacted on both sides, in which a plurality of peaks of a back contact layer extend through an oxidation layer on the absorber wafer (light scatter layer) or pass through said layer after the laser firing. U.S. Pat. Appl. 2007/0184975 A1 describes the production of a large, catalytically active surface in that a substrate is provided with catalytically active titanium dioxide needles.